18 vs 20"

[bigbear_98]

bb_98,

BC is speed dependent as you indicate, but there are changes in BC even at supersonic speeds. Litz's numbers likely includes speeds well beyond the Grendel mv, which might negate his published average BC (i.e. his published BCs are the average of multiple BCs in particular speed ranges). I have his book and modify some BCs based upon mv I have with Grendel ammo (e.g. 129 SST, I eliminated the 3000 fps BC and averaged the 1500, 2000, and 2500 fps BC).

Litz's speed ranges are (fps): 3000, 2500, 2000, 1500

He doesn't bother calculating BC into his average BC at speeds less than 1500 fps since they are not practical speeds (i.e. there is a dramatic drop in BC as the bullet approaches Mach 1 and below).

ETA:
Litz 129 SST G1 = .483 (3000 fps = .520; 2500 fps = .499; 2000 fps = .481; 1500 fps = .430)

I use G1 = .470

So the 1380 number has some significance to Todd hodnett's formulas. I'm pretty sure his philosophy is true the velocity to as close to whatever yardage your bullet will be at 1380fps. I'll have more info on this after I take his class in two weeks. The BC is constantly changing as bjorn says but right now my explanations is a simplified version.

[LRRPF52]

Velocity has a significant effect on aerodynamic drag. You will see varying BC values for projectiles provided by Sierra when you look at their site, and velocity figures will be provided with different BC's.

The real end-run around theoretical drag models in doppler radar data and compatible ballistic programming, such as Lapua's Qucik Target Unlimited. Not all bullets have doppler data available for them though.

[SMM123]

It isn't the velocity, it is the disruption of vital systems that brings the animal down. It is difficult to break enough bone to reliably anchor the animal unless one uses very large bullets. Hits on the central nervous system are really iffy and should not be counted on. That leaves arterial, hear, and venous rupture. The lungs are there too but it is difficult to cause enough direct lung damage to kill quickly.

The most reliable way to bring an animal down quickly is then to dramatically drop the blood pressure in the brain, which generally causes fainting within about ten seconds. There is a relationship between animal weight and permanent wound channel needed to have good chances for the cutting of enough major arteries or the heart to cause this to happen. Combining this relationship with empirical penetration and expansion of hunting bullets then lets one assign bullet weights to animal weights. We then get a chart that looks like this:



The method and background is described in this paper: Ideal Bullet Weight

I also assume you are designed to expand at Blackout velocities, which should make the chart valid for your bullet weights too.

Velocity must count for something. If I pick up a 220gr projectile and throw it at a Moose, the most likely death will be my own. The provided chart indicates velocities of 2000-3000fps.

The basis of my original question relates to the comment in post #10. Does 20fps really matter, or is some horrendous instability imparted during the transition from supersonic to subsonic that makes that 20fps matter?

[JASmith]

Regarding the chart. The velocity range of 2000 to 3000 fps is correct for most rifle bullets. Many pistol bullets and, I believe, some bullets for the Blackout are designed to expand well below 900 fps. Believe it or not, you won't see much change in penetration depth in the thorax region going down to those low velocities. There will be some, which is why going somewhat heavier, like to 200 - 220 grains makes sense for anchoring the animal.

Addressing the last question, twenty or even 50 fps in the muzzle velocity rarely is significant unless one is engaged in serious precision shooting.

The supersonic transition zone is actually a few hundred fps wide with the rate of change of BC changing only slowly at the boundaries. Hence one does not want to have the bullet transition through that zone. Starting at 900 fps accomplishes that. Shooting fast enough so that the bullet is above about 1300 fps at the target also does that.

I assert that most of us do not have the shooting skills to directly observe accuracy losses resulting from going through the supersonic transition. Nonetheless, it doesn't hurt (much - recoil can be an issue!) to choose loads and barrel lengths that avoids the zone.